Polymerisable compounds have been described in prior art for various purposes. For example, they can be used in polymerisable liquid crystal mixtures, which are aligned in their liquid crystal phase and subsequently polymerized in situ to give linear or crosslinked liquid crystal polymer films with uniform orientation of high quality. These films can be used as optical elements like polarisers or compensators in flat panel displays, as described for example in EP 0 397 263, EP 0 606 940 WO 97/35219, WO 98/00475, WO 98/04651 or WO 98/12584.
Polymerisable compounds have also been suggested for use in polymerised cholesteric liquid crystal films or coatings that show selective reflection of visible light and are suitable as optical films and for preparation of liquid crystal pigments, as described for example in WO 97/30136. Another important field of use are security markings as described for example in U.S. Pat. No. 5,678,863, or hot stamping foils as described for example in GB 2,357,061.
For the above applications, usually mixtures of two or more polymerisable compounds are used, as they have lower melting points than single compounds. It is especially desired to have available polymerisable mixtures exhibiting a liquid crystal phase, preferably a nematic or chiral nematic phase, at room temperature, so that it is possible to carry out alignment and polymerisation at low temperatures. Therefore, it is advantageous if the polymerisable compounds do not suppress or in the ideal case do enhance liquid crystalline phase behaviour of the mixture.
Further to the use as optical films in displays, polymerisable compounds have been suggested for use in the active, switchable layer of a liquid crystal display.
For example, displays are known which are switched between transparent and scattering states and comprise a low molar mass liquid crystal (LC) medium and a phase-separated polymerised liquid crystal material, like for example PDLC (polymer dispersed liquid crystal) displays as described in WO 93/22397, or polymer gel or polymer network displays of the scattering type, as described in U.S. Pat. No. 5,538,768, U.S. Pat. No. 5,543,075 or EP 0 451 905.
Furthermore, displays are known wherein a low molar mass LC medium is switched between two non-scattering states, like conventional displays of e.g. the TN or STN (twisted nematic, supertwisted nematic), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane-switching) mode, and which further comprise a polymerised liquid crystal material in order to create a multidomain structure for improving the contrast at wide viewing angles or to stabilise the different switching states for reducing the driving voltage and switching times. Such displays are described for example in U.S. Pat. No. 5,189,540, U.S. Pat. No. 6,177,972, EP 0 903 392, and Hasebe et al., Jpn. J. Appl. Phys. 1994, 33, 6245.
For use in the switchable layer of LC displays the polymerisable compounds are typically mixed with a low molar mass LC medium.
Therefore it is required that the polymerisable compounds do not negatively affect the properties, like the liquid crystal phase range and birefringence, of the LC medium. For use in switchable displays that are not of the scattering type, and where the polymerised material is not macroscopically phase separated from the low molar mass LC medium, the polymerisable compounds should further show good miscibility with the LC medium.
The polymerisable mesogenic compounds described in prior art, however, do often exhibit high melting points, show poor solubility in low molar mass LC media or tend to negatively influence the liquid crystal properties of LC media.
Thus, there is a demand for polymerisable compounds with low melting points which can be used for the preparation of oriented LC polymer films, which are suitable for use in the active layer of switchable LC devices in a mixture with low molar mass LC media, and which have properties adapted to those of the LC media, and show good solubility in the LC media.
Furthermore, regarding the broad range of applications for polymerisable compounds it is desirable for the expert to have available further compounds of this type which are easy to synthesize and fulfill the various requirements as described above.
It was an aim of the invention to provide polymerisable compounds with the advantageous properties mentioned above, thus extending the pool of polymerisable compounds available to the expert. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It was found that these aims can be achieved by providing polymerisable monocyclic compounds according to the present invention.